Rabbits are not rodents, but are a different kind of gnawing mammal called a lagomorph. Lagomorphs differ from rodents chiefly in that they have four incisors rather than two in the upper jaw. Hares, rabbits and small lemming-like creatures called pikas all belong to this group.
Lagomorphs differ from rodents in several other ways. Their incisors, the flat front teeth that they use to gnaw, are entirely covered in enamel. Rodents only have tooth enamel on the front surface of their incisors. Many rodents are omnivorous, but lagomorphs are incapable of digesting meat. Rabbits in particular have a digestive system more similar to cows than to rats. Cows regurgitate food to digest it a second time, while rabbits eat their own partially digested feces. Both cows and rabbits consume partially digested food in order to gain more nutrition from the vegetable matter that they eat.